Question

I am learning C, and after starting out learning C++ as my first compiled language, I decided to “go back to basics” and learn C.

There are two questions that I have concerning the ways each language deals with functions.

Firstly, why does C “not care” about the scope that functions are defined in, whereas C++ does?

For example,

int main()
{
    donothing();
    return 0;
}

void donothing() { }

the above will not compile in a C++ compiler, whereas it will compile in a C compiler. Why is this? Isn’t C++ mostly just an extension on C, and should be mostly “backward compatible”?

Secondly, the book that I found (Link to pdf) does not seem to state a return type for the main function. I check around and found other books and websites and these also commonly do not specify return types for the main function. If I try to compile a program that does not specify a return type for main, it compiles fine (although with some warnings) in a C compiler, but it doesn’t compile in a C++ compiler. Again, why is that? Is it better style to always specify the return type as an integer rather than leaving it out?

Thanks for any help, and just as a side note, if anyone can suggest a better book that I should buy that would be great!

Answer 1

Firstly, why does C “not care” about the scope that functions are defined in, whereas C++ does?

Actually, C does care. It’s just that C89 allows implicitly declared functions and infers its return type as int and its parameters from usage. C99 no longer allows this.

So in your example it’s as if you had declared a prototype as

int dosomething();

The same goes for implicit return types: missing return types are inferred as int in C89 but not C99. Compiling your code with gcc -std=c99 -pedantic-errors yields something similar to the following:

main.c: In function 'main':
main.c:2:5: error: implicit declaration of function 'donothing' [-Wimplicit-function-declaration]
main.c: At top level:
main.c:5:6: error: conflicting types for 'donothing'
main.c:2:5: note: previous implicit declaration of 'donothing' was her

For the record, here’s the code I’ve used:

int main() {
    donothing();
    return 0;
}
void donothing() { }